1. Field of the Invention
The present invention relates to a process and headbox system for improving the consistency cross-direction profile of a fiber web, e.g., a paper or cardboard web, in a wet process. The process includes distributing, via the headbox, a fiber suspension across a width of the machine onto either a wire or between two wires, and at least partially adjusting a cross-direction basis weight that is sectionally variable across the machine width. The adjustment of the cross-direction basis weight includes mixing a first suspension having a first solid content and a first percentage composition of solids content with a second suspension having a second solid content, which is lower or higher than the first solid content. A second percentage composition of the solids content of the second suspension is different from the first percentage composition.
2. Discussion of Background Information
U.S. Pat. No. 5,560,807 discloses sectionally feeding a suspension with additives, chemical admixtures, or weighting agents in a regulated manner in a diaphragm-controlled headbox in the area of a lateral distributor. In this manner, with respect to the z-direction of the resulting web or paper, the concentration of admixtures can be adjusted and the feed of the admixtures can take place as close as possible to the outlet of the headbox. Thus, a change in the admixtures due to an excessively long dwell time in the pulp slurry can be prevented.
Commonly assigned German patent disclosure document DE 40 19 593 A1 discloses a sectional consistency-regulated headbox having two sets of individual feeds for two pulp slurries of differing solids content which are distributed across the machine width. Preferably, pulp stock and backwater are used in this process. When the basis weight cross-direction profile of the paper web deviates at a specific position of the web width, the concentration CM of the sectional flow corresponding to this specific position is corrected by changing the quantitative proportion of the regulating flows of differing solids content Q.sub.H /Q.sub.L fed to a mixer. This type of basis weight cross-direction profile regulation is very advantageous with respect to a simultaneously good fiber orientation cross-direction profile, and yields very good results with regard to basis weight cross-direction profile. However, as requirements concerning paper quality are constantly increasing, it is clear that papers manufactured in this way can exhibit striped wild formations with respect to their printability despite even fiber orientation and even basis weight. Detailed studies have shown that this uneven printability is attributable to an uneven composition of the paper.
U.S. Pat. No. 5,401,363 and its counterpart European patent document EP 0600 251 B1 discloses sectionally feeding differing quantities of a retention agent to the headbox across the machine width to eliminate an uneven consistency crossdirection profile of a paper web arising through wear on the wires of differing intensity. However, a process of this type is unsuccessful if the original consistency of the pulp slurry is already uneven across the machine width.